coffeeroomfan: I can not tell you for sure in your specific case, but in my experience – yes, a person’s metabolism changes after having a baby. And each pregnancy is a new story.
I’m not sure how long ago you gave birth. If your baby is still an infant, then you still have the raging hormones and in a couple of months, you might be able to go back to your old way of eating (and drinking Coke) with no problem. But as of now, when you are trying to lose weight, cutting out the soda is an easy way to cut your calorie intake.
I give you a brocha: You should go back to your size 6 quickly and easily! Oh – and you should be written for a good gebentched yor!